It has heretofore been known to use a gradient function material as a heat resistant material in locations where the difference between inner and outer temperatures is very large, e.g., as a surface layer material for a space shuttle.
Conventional gradient function materials have been produced by either making, laminating, pressing, and thereafter firing a number of green sheets which have slightly different compositions, or an evaporation process such as CVD or the like.
However, a gradient function material which is produced by laminating green sheets does not have a completely continuous range of compositions, but has compositions varying stepwise transversely thereacross. Therefore, such a gradient function material fails to fully perform the desired function thereof.
A gradient function material which is manufactured by an evaporation process such as CVD or the like can have a completely continuous range of compositions. However, since the thickness of a film that can be deposited in one evaporation process is very small, it is very difficult from the standpoint of cost and technology to obtain a gradient function material having a required thickness.
There have been proposed methods of achieving a completely continuous range of compositions and maintaining a desired material thickness as disclosed in Japanese laid-open patent publications Nos. 3-165832 and 3-274105.
According to the method disclosed in Japanese laid-open patent publication No. 3-165832, a first slurry containing Ti particles or the like and a second slurry containing SiC particles or the like are prepared and supplied to a filtering tank which has a filter, while continuously varying the mixing ratio of the first and second slurries. When the slurries are drawn by a vacuum pump, a cake having a gradient composition is formed on the filter. The cake is then formed to shape while at the same time it is being dehydrated, after which the shaped cake is fired.
According to the method shown in Japanese laid-open patent publication No. 3-274105, a slurry is prepared which contains a plurality of types of particles having different particle size distributions. The slurry is put in a container made of film (corresponding to the filter disclosed in Japanese laid-open patent publication No. 3-165832), and a cake having a gradient composition is formed in the container by centrifugal separation or sedimentation. The cake is then formed to shape while at the same time it is being dehydrated, after which the shaped cake is fired.
However, the apparatus which are required are complex and expensive because the vacuum pump and a rotating device for generating the centrifugal force are necessary.
The gradient layer that is formed is of an increased total thickness of several millimeters as necessitated by the handling of the cake. Therefore, the gradient layer has a large heat capacity and is of a structure vulnerable to a heat shock.
FIG. 24 is an enlarged view showing the manner in which particles are attracted when a filter or a container of film is used. Particles 202 are concentrated on holes 201 in a filter 200, creating gaps in regions other than the holes 201. Therefore, a cake that is produced has a water content of 30% or more. Since such a cake cannot directly be fired, it has heretofore been customary to dehydrate the cake, making the manufacturing process complicated.
When a cake is dehydrated and dried, the product tends to crack and shrinks to a very large degree due to dehydration and drying. Therefore, it is necessary to cut out the product after the cake is dehydrated and dried.
FIG. 25 is a cross-sectional view of a gradient layer 204 which is formed according to a conventional method. Inasmuch as particles are attracted under suction forces applied in a constant direction in the conventional method, the laminae of the gradient layer 204 are arranged as horizontal stripes, and are liable to peel off because they are superposed too orderly in the transverse direction.
Furthermore, gradient function materials manufactured according to the conventional methods have shapes that are limited to simple shapes. Specifically, according to the conventional methods, since water has to be removed from a cake in a pressing step, the gradient layer would be deformed out of an orderly configuration when dehydrated if it were of a complex shape.
Consequently, the prior methods proposed in Japanese laid-open patent publications Nos. 3-165832 and 3-274105 suffer the following drawbacks:
a) As the vacuum pump and the rotating device for generating the centrifugal force are necessary, the apparatus is complex and expensive.
b) The gradient layer which is formed is of a large thickness of several millimeters as necessitated by the handling of the cake. Therefore, the gradient layer has a large heat capacity and is of a structure vulnerable to a heat shock.
c) If a filter or a film is used, the water content of a cake is large (30% or more). Because such a cake cannot directly be fired, it is indispensable to dehydrate the cake, making the manufacturing process complex.
d) When a cake is dehydrated and dried, the product tends to crack and shrinks to a very large degree due to dehydration and drying. Therefore, it is necessary to cut out the product after the cake is dehydrated and dried.
e) Inasmuch as particles are attracted under suction forces applied in a constant direction, the laminae of the gradient layer are arranged as horizontal stripes, and are liable to peel off because they are superposed too orderly in the transverse direction.
f) Gradient function materials manufactured according to the conventional methods have shapes that are limited to simple shapes. Specifically, according to the conventional methods, since water has to be removed from a cake in a pressing step, the gradient layer would be deformed out of an orderly configuration when dehydrated if it were of a complex shape.
g) The composition varies such that a certain component either increases or decreases in the transverse direction. Therefore, it has not been conventionally possible to obtain any gradient function materials which have a composition peak intermediate in their transverse direction.